1. Field of the Invention
The present invention relates to a cathode-ray tube such as a monochrome or color image picture tube, and more particularly to a cathode-ray tube wherein a phosphor screen is coated on the inner surface of a face plate and has a plurality of regions, which are scanned independently of one another.
2. Description of the Related Art
Recently, various research has been made in high-quality broadcasting and high-resolution picture tubes having a large screen designed for the high-quality broadcasting. In general, in order to achieve high resolution of a picture tube, the spot diameter of an electron beam on a phosphor screen must be reduced. For this purpose, in the prior art, the structure of an electrode of an electron gun has been improved, or the caliber and/or length of the electron gun has been increased. However, satisfactory achievement has not been obtained. The main reason is that the distance between the electron gun and the phosphor screen increases in accordance with the increase in size of the picture tube and the magnification of the electron lens increases excessively. Accordingly, in order to achieve high resolution, it is important to shorten the distance (depth) between the electron gun and the phosphor screen. In addition, when the deflection angle of an electron beam is increased, the difference in magnification between the center area and peripheral area of the phosphor screen increases. Thus, wide-angle deflection is not advantageous for achieving high resolution.
Under the circumstances, in the prior art, there is known a method of arranging a plurality of independent small-sized picture tubes, thereby constituting a high-resolution, large screen. This kind of method is effective for large-scale screen displays having a large number of divided regions, which is designed for outdoor installation. However, when this method is applied to middle-scale screen display (e.g. the screen size is about 40 inches), connection portions between the divided regions of the screen are conspicuous, resulting in low-quality images. Thus, when the display formed by this method is used a household TV receiver or computer-aided design (CAD), the connection portions on the screen are a serious defect.
On the other hand, U.S. Pat. No. 4,714,856 discloses a picture tube wherein a plurality of independent picture tubes are continuously arranged and the screens of these picture tubes are integrated. According to that design, the picture tube has an integrated phosphor screen, a vacuum envelope is constituted by a face plate having an inner surface coated with a phosphor screen, a rear plate is opposed to the face plate, funnels are adjacent to the rear plate, and necks are provided on the funnels. The face plate is formed of glass, and the rear plate is formed of glass or metal.
In the case of this structure of the envelope, however, if the screen surface becomes broader, it is necessary to increase the thickness of the face plate or rear plate in order to withstand the load of atmospheric pressure (external pressure). In addition, it is necessary to provide the face plate with a high curvature in the tube axis direction. As a result, the weight of the envelope becomes considerably heavy, and moreover the screen of the picture tube with the face plate having such a high curvature in the tube axis direction cannot be viewed clearly. In addition, the distance between the phosphor screen and the electron gun sealed within the neck increases, and the magnification of the electron lens is adversely affected.
In order to solve the problems described above, it is necessary to provide the face plate, in particular, with a relatively flat, large area. In this case, it is also necessary to provide the envelope with support means for supporting the load of atmospheric pressure applied to the face plate.
A picture tube with this support means is disclosed, for example, in Published Unexamined Japanese Patent Application (PUJPA) No. 64-10553. According to that picture tube, an elongated plate-shaped support member is provided between a flat face plate and a rear plate as the support means for withstanding the atmospheric pressure applied to the evacuated flat envelope. There is another example wherein a needle-shaped support member is provided, in addition to that support member.
It is desirable that the support means be situated outside the locus of the electron beam, if possible, in order to prevent a shadow from being thrown on the phosphor screen when the electron beam collides with the support means, and that the area of contact between the support means and the phosphor screen be reduced as much as possible, thereby reducing the area of non-light-emitting portion produced by this contact.
There are many problems, however, in the structure wherein the elongated plate-shaped or needle-shaped support members are provided between the face plate and the rear plate, which are used withstand the load of the atmospheric pressure applied to the evacuated flat envelope. For example, the elongated plate-shaped support members have problems: (a) processing precision, (b) strength against load, (c) fixing method, (d) cost, etc. Further, the needle-shaped support members have a problem of (e) increase in number of used support members.
The above picture tube is effective, in particular, when the screen size is large, but various problems occur when that picture tube is applied to a cathode-ray tube for displaying color images, i.e. a color picture tube having therein a shadow mask serving as a color selection electrode.
First, there is a problem in the method of attaching the shadow mask. Specifically, in the case of a conventional color picture tube having a spherical face plate, and a spherical shadow mask. In this case, by fixing a peripheral portion of the shadow mask to a metallic frame (mask frame), practical mechanical strength can be given to the shadow mask and it becomes easy to situate the shadow mask in a predetermined positional relationship with the phosphor screen, which is formed on the inner surface of the face plate. However, in the case of a flat face plate, the shadow mask must also be flattened, and therefore the mechanical strength of the shadow mask is low. Accordingly, that shadow mask cannot easily be situated in a predetermined positional relationship with the phosphor screen formed on the inner surface of the face plate, only by fixing the peripheral portion of the shadow mask, as in the prior art.
In general, regarding a flat shadow mask or a cylindrical shadow mask which has a curvature only in one direction, sufficient mechanical strength is given to the shadow mask by fixing it to a robust metallic frame with a tensile force applied to the shadow mask, and the shadow mask is attached to the face plate via this metallic frame. In this method, however, if the size of the shadow mask increases in accordance with the increase in screen size, the tensile force applied to the shadow mask must be increased accordingly. Consequently, a more robust metallic frame is required. In this case, not only the weight of the entire picture tube increases, but also the attaching means for attaching the shadow mask to the face plate via the metallic frame must have a special structure. Furthermore, a sufficient space for providing the attaching means is required.
Secondly, there is a problem in precision of arrangement of the shadow mask. A phosphor screen of a regular color picture tube is formed on the inner surface of a face plate by a photo-engraving method, by using a shadow mask built into the color picture tube on the basis of a projection image formed through the shadow mask. Thus, if a distance (q-value) between the shadow mask and the inner surface of the face plate departs from a predetermined value, the arrangement pitch of phosphor layers is affected but the continuity of the entire phosphor screen is not affected. On the other hand, in the case of a color picture tube wherein an integrated phosphor screen has a plurality of regions which are scanned independently of one another, the continuity of the phosphor screen, i.e. the continuity of images projected onto adjacent regions of the shadow mask, is affected by the q-value. More specifically, when the q-value is greater than a predetermined value, projected images on adjacent regions overlap one another; when the q-value is less than a predetermined value, a gap is produced between projected images on adjacent regions.
In addition, when a phosphor screen is formed by a so-called master mask method, (i.e. by using an exposure mask or a dry plate), the distance between the shadow mask and the inner surface of the face plate on which the phosphor screen is formed must be exactly determined. If the q-value is not exact, an electron beam does not land on a predetermined phosphor layer, rasters between adjacent regions overlap one another, or a gap is produced between the rasters. Further, the required precision of the q-value is about 0.05 mm, though it depends on the horizontal deflection angle or the arrangement pitch of the shadow mask. As can be seen from the fact that the required manufacturing precision of the conventional color picture tube is about 0.5 mm, very high precision is required of the q-value. In other words, it is difficult to provide the shadow mask with a high precision by the conventionally known means.
For example, International Patent Application PCT/US87/02869 discloses a picture tube wherein a shadow mask is supported on the inner surface of a face plate (i.e. on a phosphor screen) via attaching members. This shadow mask is supported in the state in which both end portions thereof are attached to the attaching members. In the case of this structure, however, the attaching members cannot be mounted on a central portion of the phosphor screen, in order to prevent occurrence of shadow on the phosphor screen due to collision of electron beam with the attaching members and occurrence of non-light-emitting portions. Thus, the attaching members are disposed on only outer edge portions of the phosphor screen, and only both end portions of the shadow mask are supported by the attaching members. Accordingly, the central portion of the shadow mask cannot be supported by the attaching member. Thus, in particular, when the screen size is large, the central portion of the shadow mask may easily be warped.
As a result, with this picture tube, too, it is difficult to have the shadow mask supported on the phosphor screen with high precision.